Fermentation and Fermenters
In simple terms, fermentation is the chemical breakdown of larger molecules into smaller molecules, in the absence of oxygen. This breakdown of larger molecules also results in the release of energy. For living organisms, fermentation is a way to obtain energy needed for maintenance, growth and activities, when oxygen is neither available or utilized by the organism (e.g. anaerobic bacteria). We experience the process, or more noticeably the by-products, of fermentation when we engage in intense exercise. That burning feeling in your muscles from intense exertion is the buildup of lactic acid that results from muscle tissues running out of oxygen and having to resort to fermentation to obtain the energy needed for exertion. In this case, energy is harvested from the breakdown of glucose into pyruvate which is then converted to lactic acid. For us, this is an inefficient means of obtaining energy that is only used when the more efficient aerobic respiration metabolic process is not sufficient to meet energy demands. But for certain microorganisms, fermentation is the primary means of obtaining energy.
Microorganisms
Microorganisms, or microbes, such as bacteria and yeast, lack mouths with which to ingest food. Instead, they excrete enzymes into the environment around them, which breaks down large organic molecules into smaller molecules, such as glucose, that can be absorbed and then further broken down to harvest energy. For microbes that are anaerobic, or that can live in anaerobic conditions (obligate anaerobes) fermentation is the means by which they obtain the energy to live, grow and multiply. The by-products of that fermentation are excreted back into their environment.
Fermentation By-Products
The by-products that microbial fermenters release into their environment can be useful to other organisms. Indeed, humans have even found ways of exploiting microbial fermentation to produce useful substances. Here is a list of useful substances, and the microorganism(s) that are used to produce them (1):
FOOD ITEMS
Bread, beer, wine and spirits - Saccharomyces cerevisiae (yeast)
Cheese and yogurt - lactic acid producing bacteria
Ripening of blue and Camembert-type cheese - Penicillium species (yeast)
Soy sauce - Aspergillus oryzae (yeast), Zygosaccharomyces rouxii (yeast)
Vinegar - Acetobacter species (bacteria)
AGRICULTURAL ITEMS
Fungicides - Coniothyrium minitans (yeast)
Insecticides - Bacillus thuringiensis (bacteria)
AMINO ACIDS
L-Glutamine - Corynebacterium glutamicum (bacteria)
L-Lysine - Brevibacterium lactofermentum (bacteria)
L-Tryptophan - Klebsiella aerogenes (bacteria)
FUELS AND CHEMICAL FEEDSTOCKS
Acetone - Clostriudium species (bacteria)
Butanol - Clostridium acetobutylicum (bacteria)
Ethanol - Zymomonas mobilis (bacteria), Saccharomyces cerevisiae (yeast)
Glycerol - Zygosaccharomyces rouxii (yeast)
ANTIBIOTICS
Streptomycin - Streptomyces griseus (bacteria)
Penicillins - Penicillium chrysogenum (yeast)
Cephalosporins - Acremonium chrysogenum (yeast)
Nisin - Lactococcus lactis (bacteria)
Erythromycin - Saccharapolyopora erythraea (bacteria)
Bacitracin - Bacillus lichenformis (bacteria)
Chlorotetracycline - Streptomyces aureofasciens (bacteria)
VITAMINS
B12 - Pseudomonas denitrificans (bacteria)
Beta-carotene (provitamin A) - Blakeslea trispora (yeast)
C - Acetobacter suboxydans (yeast)
Short Chain Fatty Acids
Horses, as with other mammals, lack the enzymes necessary to breakdown plant fibers. The microbes in a horse's hindgut, however, are able to produce such enzymes and digest the plant fibers. One type of by-product of this fermentation is short chain fatty acids (SCFA). Acetate, butyrate and proprionate are the most commonly produced SCFA (2). The National Research Council's, "Nutrient Requirements of Horses", 6th edition, cites a study that estimated that horses on a diet of just hay will derive 80% of their energy needs from the SCFA produced in their hindgut by microbial fermentation. It cannot be understated how important fermentation is to horses, as well as other herbivore species, that rely on microbial fermentation of plant material for energy.
Horse Feeds
Manufacturers of horse feeds include a number of microbial and microbial-derived ingredients in their feed formulations.
Yeast culture, active dry yeast
These ingredients are live cultures of microorganisms. The AAFCO regulations limit acceptable yeast cultures to Saccharomyces cerevisiae and Kluyveromyces marxianus (3). While it is questionable whether yeast can survive the heat and shearing stresses of the feed pelleting process, the inclusion of live microbes is likely intended to supplement the natural gut flora and facilitate the digestion of the feed.
Distillers dried grains with solubles
In the distillation process, yeast ferment sugars from cereal grains into ethanol. After the ethanol is removed (distilled) the remaining mash is a rich mixture of non-starch grain nutrients, non-viable yeast and digestive yeast enzymes (solubles). This ingredient provides digestive enzymes, protein, fat, fiber, vitamins and minerals. The digestive enzymes will be specific to the starch composition of the grain that was fermented. So the enzymes derived from corn distillation may not be appropriate or optimal for digestion of a grain component in the feed if different from corn, such as oats or wheat.
Dried Fermentation products, extracts and solubles
Fermentation products, extracts and solubles from specific microorganisms are largely composed of the digestive enzymes of the fermenting microorganism. Frequently used microbes include Bifidobacterium thermophilium (bacteria), Enterococcus faecium (bacteria), Lactobacillus acidophilus (bacteria) and Lactobacillus casei (bacteria). The enzymes produced, however, will be specific to the food source in the media used to grow it, which may or may not be optimal for digestion of the carbohydrate components in the feed formulation.
Vitamins and Minerals
Microbes can be cultured and used to produce vitamins such as K and B12, and organic mineral chelates such mineral-polysaccharide and mineral-amino acid complexes.
Fermentation
Fermentation is an important biochemical process. Not only is it a means of harvesting energy from organic molecules, under anaerobic conditions, but the by-products can be useful as well. Looking at the ingredient list of horse feeds you will likely find ingredients that are the microorganisms themselves or the by-product of their fermenting activity.
Industrial Microbiology is an applied science discipline within the field of Microbiology that focuses on the commercial use of microorganism in the production of useful products by fermentation, and in the degradation of organic waste and pollution (1).
Waites, W.J., Morgan, N.L., Rockey, J.S., & Higton, G. (2001). Industrial Microbiology: An Introduction. Oxford, England: Blackwell Science Ltd
NRC. (2009). Nutrient Requirements of Horses. (6th Ed.) National Academy Press, Washington D.C.
Association of American Feed Control Officials Incorporated. (2017). 2017 Official Publication. Champaign, USA. AAFCO